documents: improved reordering of documents in the tree (performance)

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2727 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
PIMCORE-1809 - Document ordering - problem with sort index

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2728 1f8fe7d8-47f0-464c-8d0a-336f4953ab05